Known in the present state of art are devices for magnetic holding of workpieces (cf. an advertising prospectus of Walker-Hagou. BV, EFE 05/83 Alterations reserved, or a textbook "Magnetic and electromagnetic jigs and fixtures in machining practice" by A. Ya. Vernikov. Moscow, Mashinostroenie Publishers, 1984, p. 56, Table 9, Para. 1,3), comprising a workpiece holding plate and a magnetic field excitation unit which incorporates magnetic cores of a soft magnetic material, mounted on a base, said cores being encompassed by direct-current coil windings and thus establishing the main poles of a magnetic system. The workpiece-holding plate is in fact a sectionalized structure made up of steel bars situated above the main poles of the magnetic system, and placed therebetween steel plates interspersed alternatively with nonmagnetic material plates.
Such a constructional arrangement of the device makes it possible to hold both large- and relatively small-sized workpieces, a feature giving the device an important advantage and providing technological versatility of the device.
However, the aforementioned composite plate is too complicated in manufacture. In addition, the plate is subject to mechanical strain and thermal effect which cause dissimilar deformation of the inhomogeneous elements making up the plate. This, in turn, leads to lost geometrical trueness and formation of microcracks which affects adversely workpiece locating accuracy and quality of machining, as well as operating reliability of the device due to possible loss of tightness of the plate.
One prior-art device for magnetic holding of workpieces (cf. an advertising prospectus of Walker-Hagou. BV for a symposium on magnetic holding appliance, Odessa, 1983, p. 10-11, FIG. 10, or a textbook "Magnetic and electromagnetic jigs and fixtures in machining practice" by A. Ya. Vernikov. Moscow, Mashinostroenie Publishers, 1984, p. 50, Table 7, Para. 1) is known to comprise a workpiece holding plate and a magnetic field excitation unit which incorporates base-mounted magnetic cores whose bottom portions are enveloped by coil windings, while their top portions or extensions thereof are located in the plate interior and are magnetically insulated from the plate with elements made of a nonmagnetic material. The plate is an all-metal structure except for those portions that accommodate the top portions of the excitation unit magnetic cores.
The fact that the top portions of the magnetic cores are located in the interior of the workpiece-holding plate makes possible for magnetic flux to flow to the workpiece being held along the shortest possible pathway and hence for attaining grater attraction forces applied to the workpiece involved.
However, all stated above holds true only for comparatively large-sized workpieces which cover at least two poles of the magnetic system. Smaller workpieces are held rather poorly on this device, whereas workpieces having a size substantially lower than the interpole distance cannot be held altogether. To provide a possibility of holding such workpieces, the interpole distance is to be reduced, which results in an increased number of magnetic cores and coils, drastically sophisticates the construction and affects adversely the reliability of the device. At the same time inhomogeneity of the plate is increased due to a closely spaced network of nonmagnetic elements, which degrades the workpiece locating and machining accuracy. Such a device is also fraught with a danger of the plate getting untight at the places where the nonmagnetic elements are located and hence of the coolant to penetrate to the coil windings.